1. Field of the Invention
This invention relates to a surface-treated metal sheet of high durability which comprises a metal sheet coated with a thermoplastic polyolefin resin and a paint for a colored metal sheet prior to its forming into a particular shape, or its use, is used mainly as a material for the construction of an outdoor structure, and has high levels of corrosion resistance, weatherability and formability, and a process for manufacturing the same.
2. Description of the Prior Art
It has hitherto been usual to manufacture a surface-treated and coated metal sheet as a material for an outdoor structure; or article by employing the continuous application of a paint, the continuous lamination of a film, or the continuous hot-melt coating on a metal sheet, such as a hot dipping galvanized steel sheet conforming to JIS G-3302, a zinc electroplated steel sheet conforming to JIS G-3313, a steel sheet galvanized with a 5% Al--Zn alloy which conforms to JIS G-3317, a steel sheet plated with a 55% Al--Zn alloy, a steel sheet plated with aluminum, an aluminum sheet, or a stainless steel sheet, as described below:
(1) Continuous Application of a Paint:
A metal sheet which has been given surface treatment is continuously coated with a paint by, for example, a roll coater to make a two-coat and two-bake colored metal sheet;
(2) Continuous Lamination of a Film:
A metal sheet which has been given chemical surface treatment is undercoated with, for example, an adhesive, and after it has been dried, a resin film is continuously laminated thereon to make a laminated metal sheet; and
(3) Continuous Hot-Melt Coating:
The chemically treated surface of a metal sheet is undercoated with an epoxy resin paint, and is continuously coated with a polyolefin resin and a modified polyolefin resin extruded from an extruder through a T-die to make a resin-coated metal sheet.
The surface-treated and coated metal sheet manufactured as hereinabove described has corrosion resistance, weatherability, formability, decorative property, etc., and is stable in quality. Moreover, it does not call for any further coating after its forming into a particular shape, or its use at the site of construction. Therefore, it has been used widely as a material for the construction of an outdoor structure, etc.
While any such surface-treated and coated metal sheet is required to have certain properties including weatherability, corrosion resistance, formability and scratch resistance, very high levels of such properties have recently come to be required of the sheet, so that it may withstand exposure to a severe environment, including a change of environment caused by acid rain, particularly insofar as it is used to construct a structure which is required to be maintenance-free.
As far as weatherability is concerned, it has been found that a fluororesin paint is suitable for maintenance-free application. A steel sheet coated with a thick film of vinyl chloride and a steel sheet coated with a polyethylene resin have, for example, been used as materials having a high level of corrosion resistance.
It has, however, been found that a steel sheet coated with a fluororesin paint and having excellent weatherability as stated above is easily scratched during its forming, such as bending or roll forming, or during its handling for use at the site of construction, since a film of the paint with which it is coated has a thickness which is usually as small as from 25 to 35 microns, and a low degree of hardness, and that the sheet is likely to have a lower degree of corrosion resistance in its scratched, or formed portion. Therefore, it has usually been necessary to employ a protective film, or repair any scratched portion after the work of forming. Studies have been made to solve those problems and form a coating film of a fluororesin paint having a high degree of hardness and thereby achieving an improved scratch and wear resistance without the sacrifice of the formability of the coated metal sheet, or the adhesion of the film, while retaining the advantage of the fluororesin paint. As a result, it has been proposed that glass fibers be added to a fluororesin paint with which a metal sheet is coated, as taught in Japanese Patent Publication No. Sho 60-7946 entitled "Process for Manufacturing a Coated Metal Sheet of High Durability", or Japanese Patent Application laid open under No. Sho 61-237636 entitled "Coated Steel Sheet".
According to the disclosure of those publications, it is preferable for forming a good coating film that the glass to be added to the paint in the form of surface-treated fibers.
We, the applicant of this application, have, however, found that even a film having a high degree of hardness as suggested above is not sufficient for protecting the coated metal sheet from being scratched during its forming into a particular shape, or during its handling for use at the site of construction. We have found the rapid progress of local corrosion, or the so-called flowering, on a steel sheet coated with a fluororesin paint as a result of its exposure to an actually prevailing environment. The addition of surface-treated glass beads is preferred to prevent such rapid and local flouring.
Despite these improvements, however, it has still been the drawback of the fluororesin-coated steel sheet having a high level of weatherability that the steel surface under the coating film is scratched during its forming, or its handling for use, since the film has a thickness which is as small as from 25 to 35 microns.
A steel sheet coated with vinyl chloride, or a polyolefin resin has a high degree of corrosion resistance, since a film with which it is coated has a thickness of 150 to 300 microns which is so large that, even if any scratch may be made during the forming of the sheet into a particular shape, or during its handling for use, it terminates within the film and does not reach the steel surface, and since it is so high in formability that no cracking occur to any formed portion thereof. The energy which binds the atoms forming the molecules of any of those resins is, however, lower than the equivalent energy in the fluororesin, and lower than the energy of sunlight. Therefore, those resins undergo rapid deterioration upon exposure to an actual outdoor environment, even if measures, including the control of their molecular weights, or molecular weight distribution, and the addition of a heat or light stabilizer, may be taken to improve their weatherability.